The oxygen isotopes of water ((H2O)-O-18 and (H2O)-O-16) are tracers widely used for the investigation of Earth science problems. The tracer applications are based on the premise that the O-18/O-16 ratio of open-water evaporation (delta O-18(E)) can be calculated from environmental conditions. A long-standing issue concerns the role of kinetic fractionation, or diffusion transport, in the evaporation process. Here we deployed an optical instrument at a large lake (area 2,400 km(2)) to make in situ measurement of O-18 and D of atmospheric vapor, then determined delta O-18 and delta D of open-water evaporation using the gradient-diffusion method. Our results show a much weaker kinetic effect than suggested by the kinetic factor epsilon(k) adopted in some previous studies of lake hydrology (14.2). By incorporating into the (H2O)-O-18 isotopic mass balance of the lake a lower epsilon(k) value (about 6.2) used for ocean evaporation in global climate models, we obtain an annual lake evaporation rate that agrees with an independent eddy-covariance observation, but the rate is 72% higher than if the commonly used lake epsilon(k) value of 14.2 is applied. The applicability of this results to small lakes is uncertain and in need of field-based assessment.